Ductwork improves efficiency of counterflow two pass active heat sink

ABSTRACT

The heat removal ability of a spiral finned counterflow two pass active heat sink (“Wagner” active heat sink) is increased by eliminating a slight recirculation of heated discharge air back into the cool intake air by conducting intake air to the Wagner active heat sink with a ductworks having an orifice proximate the intake end of the heat sink. The diameter of the orifice is larger than that of the Wagner active heat sink, and if the heat sink does not extend into the orifice, then a pusher fan may be located somewhere near the entrance of the duct to fill it with pressurized air that then exits from the orifice to form a curtain of cool air around the outside of the Wagner active heat sink. This curtain of cool air replaces the certain amount of heated discharge air that otherwise recirculates back into the input of the heat sink. If the ductwork can be extended toward the Wagner active heat sink, or the heat sink moved toward the orifice, such that heat sink penetrates the orifice and is partly inside the ductwork by an amount related to the geometry of the fan within the Wagner active heat sink, then the pusher fan can be eliminated in favor of the suction provided by the Wagner active heat sink itself.

REFERENCE TO RELATED APPLICATION

[0001] The subject matter of this Application is related to thatdisclosed in U.S. Pat. No. 5,785,116 entitled FAN ASSISTED HEAT SINK,filed by Wagner on Feb. 1, 1996 and issued on Jul. 28, 1998. That Patentdescribes a particular type of internal fan heat sink formicroprocessors, large power VLSI devices and the like, that dissipate asufficient amount of power to require a substantial heat sink. Theinstant invention pertains to a manner of using that same type ofinternal fan heat sink, which heat sink has a number of uniqueproperties that do not readily lend themselves to summary description:it is not a garden variety heat sink with a fan grafted onto it. Forthis reason U.S. Pat. No. 5,785,116 is hereby expressly incorporatedherein by reference, so that all the unique properties of that activeheat sink, including its manner of operation and manufacture, will befully available for the understanding of this Disclosure.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits are becoming more and more powerful all thetime. Not only is this true in the sense that they do more, and do itfaster (e.g., in the field of microprocessors and FPGA's—FieldProgramable Gate Arrays), but these newer parts dissipate amounts ofpower that were unimaginable just a few years ago. For example, thereare parts under development that will dissipate one hundred and thirtywatts and will need to get rid of the attendant heat through a surfacearea of about one square inch. There are exotic methods of heat removalthat are possible, including heat pipes, chilled water cooling and evenactual refrigeration. In the main, these techniques are cumbersome orexpensive, and are not suitable for high volume commercial applicationsin modestly priced retail equipment, such as personal computers andworkstations.

[0003] The active (meaning fan assisted) heat sink described in aboveincorporated Patent to Wagner was developed to deal with this situation.It is a heat sink having a spiral of fins that surround a fan around itscircumferential periphery and are in its discharge path. This makesWagner's active heat sink a two pass device, since the design draws aportion of its air in through the periphery (one pass) and thendischarges it through more fins (second pass). It is a counter flowdevice, since the path of heat flow is generally opposite to thedirection of air flow, so that as air is heated through contact with thefins it encounters still warmer fins as it continues along its path.This ensures greater heat transfer by maintaining temperaturedifferential between the cooling air and the fins that are to give uptheir heat to the air. In addition, Wagner's active heat sink has anumber of other desirable properties, such as low noise and an absenceof extra mating surfaces that interfere with heat flow.

[0004] The preceding several sentences are a brief description ofWagner's active heat sink, but it is probable that, unless the readerhas actually seen one, he or she will not have a completely satisfactorymental image of just what such a fine active heat sink really lookslike. We can cure that by including certain of the figures from theWagner Patent, which we have done. However, that still leaves us withthe problem of a nice tidy way to refer to it: “spiral finnedcounterflow two pass active heat sink” is accurate as far as it goes,but is also pretty cumbersome. Various heat sinks of this design are onthe market, offered by Agilent Technologies, Inc. under the trade name“ArctiCooler”, but it would be a risky business to rely on that, sincewe can't be sure what that term will eventually come to encompass. So,we will do as we have already begun to do above: we shall call the kindof fan-assisted heat sink described in the Specification of the WagnerPatent a “Wagner active heat sink”, or depending upon the grammaticalneeds at the time, “Wagner's active heat sink”. By availing ourselves ofthis coined phrase, we shall avoid much inconvenience. On the principlethat whatever makes for shorter sentences is good, when it is entirelyclear that we are indeed referring to a Wagner active heat sink, weshall fell free to call it a “heat sink” as a further simplification.

[0005] It will, of course, be appreciated that as the Wagner active heatsink gains further acceptance and additional needs and applicationsdevelop, the exact size, relative shape and so forth will evolve overtime. Thus, there are already small ones, medium and large sizes, andextra heavy duty ones, etc. Thus, it will be understood that thespecific examples shown in U.S. Pat. No. 5,785,116 (Wagner) are merelyillustrative of a general class of active heat sinks (Wagner active heatsinks), and such specific details as the number of fins, whether theyare straight or spiral, their thickness compared to their height, thenumber of blades on the fan, whether the thing is tall or squat, etc.,are not determined by our meaning of the term “Wagner active heat sink”.

[0006] To continue, then, as good as the Wagner active heat sink is, itis still the case that anything that can be done to enhance efficiencyis desirable, since the wattages to be dissipated are increasing to sucha large degree. One way to get an active heat sink that handles moreheat is to make it bigger, but it would be better if there were a way toget an existing one to handle more heat without making it bigger. Whatto do?

SUMMARY OF THE INVENTION

[0007] A solution to the problem of increasing the heat removal abilityof a Wagner active heat sink is to eliminate a slight recirculation ofheated discharge air back into the cool intake air. This is accomplishedby conducting intake air to the Wagner active heat sink with a ductworkshaving an orifice proximate the intake end of the heat sink. Thediameter of the orifice is larger than that of the Wagner active heatsink, and if the heat sink does not extend into the orifice, then apusher fan may be located somewhere near the entrance of the duct tofill it with pressurized air that then exits from the orifice to form acurtain of cool air around the outside of the Wagner active heat sink.This curtain of cool air replaces the certain amount of heated dischargeair that otherwise recirculates back into the input of the heat sink. Ifthe ductwork can be extended toward the Wagner active heat sink, or theheat sink moved toward the orifice, such that heat sink penetrates theorifice and is partly inside the ductwork by an amount related to thegeometry of the fan within the Wagner active heat sink, then the pusherfan can be eliminated in favor of the suction provided by the Wagneractive heat sink itself.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a top perspective view of a (prior art) Wagner activeheat sink;

[0009]FIG. 2 is a sectional view of the spiral finned portion of theheat sink of FIG. 1, with the fan removed for clarity;

[0010]FIG. 3 is a side view of an installed Wagner active heat sink,showing the directions of airflow, and including the undesirablerecirculation of heated discharge air into the intake;

[0011]FIG. 4 is a simplified side view of the Wagner active heat sink ofFIG. 1 deployed in conjunction with a first pressurized ductwork thatdispels the recirculation of FIG. 3 with a curtain of cool air;

[0012]FIG. 5 is a simplified side view of the Wagner active heat sink ofFIG. 1 deployed in conjunction with a second pressurized ductwork thatdispels the recirculation of FIG. 3 with a curtain of cool air; and

[0013]FIG. 6 is a simplified side view of the Wagner active heat sink ofFIG. 1 deployed in conjunction with a shroud or an unpressurizedductwork that serves as a baffle to prevent the recirculation of FIG. 3while at the same time supplying by suction from the heat sink itselfcool air to the input of the heat sink.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] Refer now to FIG. 1, wherein is shown a top perspective view of aprior art Wagner active heat sink 1. There is an annular ring 2 ofspiral cooling fins, preferable of aluminum, within the center of whichis mounted a fan 3. Not shown is the IC (Integrated Circuit) or otherdevice that is to be cooled. It would be in contact with the undersideof the heat sink, directly beneath the hub of the fan.

[0015] Now refer briefly to FIG. 2, which is a sectional view of thespiral finned portion of the heat sink of FIG. 1, with the fan 3 removedfor clarity. Note the shelf 12, which is somewhat below the bottom ofthe fan blades are. Somewhat above this shelf (at about one fourth ofthe way up the height of the fan blades) is a boundary (33) thatseparates where intake air is drawn into the heat sink and exhaust airis discharged.

[0016] The airflow situation for a Wagner active heat sink is shown inmore detail in FIG. 3. In that figure a heat sink 1 and device 9 to becooled are mated together, and the combination is mounted upon orcarried by, for example, a printed circuit board 7. Arrows 4 indicateintake air that enters the top of the heat sink 1. Arrows 5 indicateadditional intake air that enters the upper sides of the heat sink 1.Arrows 8 indicate the paths the intake air from arrows 4 and 5 followonce inside the heat sink 1. Arrows 6 indicate the path of heated airthat is discharged from the heat sink. Now for the bad news. Some amountof heated discharge air can flow along paths indicated by arrows 10 and11 to join the intake air. This is termed “recirculation”, and isundesirable because it raises the temperature of the intake air enteringthe heat sink, and diminishes its efficiency. In general, the amount ofrecirculation is determined by the degree of obstruction in the path ofthe discharged air. In an ideal case the amount of recirculation isslight, and the efficiency reduction might be only 5% or 10%. If thedischarge path is quite cluttered with large obstructions (e.g., otherheat sinks, etc.) then the efficiency reduction might be as large as50%. When the amount of power being dissipated is large, then even asmall fraction of that can be significant amount of power.

[0017] Refer now to FIG. 4, wherein is shown a simplified side view 23of a Wagner active heat sink 1 deployed in conjunction with apressurized ductwork 13 that dispels the recirculation of FIG. 3 with acurtain of cool air 19. In particular, an air duct 13 has a nozzle ororifice 17 that is slightly larger in diameter than the heat sink 1, andwhich is disposed just above to top of the heat sink. The ductwork 13,which may be of any suitable material (e.g., stamped, rolled or foldedsheet metal, extruded plastic, etc.), is pressurized with air having apositive pressure relative to the discharge 20 from the heat sink 1. Oneway to accomplish this is with a fan 16 located at a distal end of theduct 13, and which draws in cool air 14 and creates the pressure insidethe duct. Ducted air flows generally along the paths indicated by arrows15 until it reaches the heat sink. Some of that air enters the top ofthe heat sink 1 as indicated by arrows 21, some of it flows out of thenozzle 17 to travel along the top outer surface of the heat sink to bedrawn in as intake air, as indicated by arrows 18. Another portion ofthe ducted airflow 15 continues to travel downward along the outersurface of the heat sink 1, following a path indicated by arrows 19.This latter airflow along the path of arrows 19 is a curtain of cool airthat blocks the detrimental recirculation of arrows 10 and 11 of FIG. 3.There may be some airflow in the direction of arrows 22, but it does notpenetrate the curtain 19 and is not drawn back into the heat sink.

[0018] The thickness of the air curtain 19 is essentially governed bythe degree by which the nozzle 17 has larger diameter than the activeheat sink 1. A preferred range of thickness for the air curtain 19 isfrom about one quarter to one half an inch. In the figure the peripheryof the nozzle 17 is shown as being slightly above the top of the heatsink 1. It is shown this way for clarity. The preferred arrangement isthat they be about even with each other.

[0019]FIG. 5 shows a situation whose arrangement 24 is generally similarto that of FIG. 4, except that the shape of the duct 25 is different. Itmay be a plenum chamber, a back side of which may be an element of thechassis of the apparatus (e.g., a computer) whose IC 9 needs cooling. Inthe case shown, pressurized cool air arrives at the nozzle or orificefrom two directions (a typical case would be that the plenum is as longand as wide as one side of the instrument chassis, so the air wouldenter radially from all directions). We have omitted the depiction ofany auxiliary fans (e.g., 16 in FIG. 4) that would produce thispressurized air. Also, the figure suggests that airflow within theplenum is in both directions toward the heat sink. Suppose there were anextended plenum and more than one heat sink. Then airflow could be inone direction along the plenum, which air is then supplied to thedifferent heat sinks in turn.

[0020] Now consider a slightly different case, where it is undesirableor otherwise impractical to provide a source of air that is at positivepressure with respect to the discharge of a Wagner active heat sink, yetit is still desirable to reduce or eliminate recirculation. Anarrangement 27 for dealing with that situation is depicted in FIG. 6. Anozzle or orifice of a duct (similar to 25 or 13, but not shown) orperhaps just shroud extending into a region of cool air suitable for useas intake air, any of which possibilities are indicated in the figure byreference character 28, has a diameter larger than that of the heatsink, and encloses an upper portion of the heat sink 1 that is generallyabove location 33. Recall that is at about this height that intake airis separated from discharge air. What happens is this. Airflow 29 isdrawn into the top of the heat sink 1, as one would expect. Arrows 18indicate the extent of airflow that is drawn into the side of the heatsink 1, which ends at location 33. The proximity of flow along paths 18,in conjunction with a low pressure region (according to Bernoulli'sprinciple) created by discharge flow along arrows 20, draws a curtain 30of cooling air down along the outside of the heat sink. Arrows 31 and 32indicate the paths recirculation would need to take if they were tooccur. Arrows 32 indicate paths that air might take to re-enter the heatsink 1 above the level of line 33. Such air is instead swept along bythe curtain 30. Even if some of that air (32) mixes with curtain 30 andthen tries to re-enter above line 33 along path 31, it is blocked by theshroud or nozzle 28.

[0021] There is yet another way in which airflow for the cases of FIGS.4, 5 and 6 can be induced (with or without the fan 16 of FIG. 4). Thisother way is to locate an exhaust fan 34 within the chassis. (We showthis in conjunction with FIG. 6, but it will be appreciated that itapplies to FIGS. 4 and 5, as well.) If the air pressure at the top ofthe heat sink 1 is essentially at the same pressure as the exterior ofthe chassis, (especially possible if the ductwork that is there is forthat purpose), then the exhaust fan creates a pressure differential thatproduces the air curtain 19.

I claim:
 1. In an active heat sink having fins surrounding a fan thatdraws intake air into an end of the fan as well as through intakeportions of the fins surrounding the fan, and which discharges airthrough adjacent discharge portions of the fins, a method of preventingdischarged air from recirculating as intake air, the method comprisingthe steps of: ducting air to be used as intake air to a nozzle proximatethe end of the fan; pressurizing the ducted air above the pressure ofthe discharged air; and directing a curtain of pressurized air from thenozzle over both the intake and the discharge portions of the fins. 2.In an active heat sink having fins surrounding a fan that draws intakeair into an end of the fan as well as through intake portions of thefins surrounding the fan, and which discharges air through adjacentdischarge portions of the fins, a method of preventing discharged airfrom recirculating as intake air, the method comprising the steps of:ducting air to be used as intake air to a nozzle having an interiorsurface that surrounds the intake portions of the fins and that has aperiphery proximate a boundary separating the intake portions of thefins from the discharge portions of the fins; and inducing, withBernoulli's principle and the reduced pressure of the discharged air, acurtain of air from the nozzle to flow over the discharge portions ofthe fins.